Electronic components are provided with transmission lines for transmitting electric signals. With the advent of highly advanced information in recent years, alternate current (AC) electric signals transmitted by transmission lines have been shifting their frequency bands to higher frequency bands. For example, communication frequency bands in mobile information terminals range from several hundreds of MHz to several GHz. A skin effect occurs in a high-frequency transmission line which transmits AC electric signals in such a high frequency band. In the skin effect, the current density of the high-frequency signal flowing through the transmission line is higher on a surface of the transmission line and becomes lower as farther away from the surface. As the frequency of the AC electric signal is higher, the current concentrates more on the transmission line surface, whereby the AC resistance increases in the transmission line. Hence, for lowering the AC resistance in the high-frequency transmission line, it is required to attain higher electrical conductivity on the transmission line surface.
As an example of methods for lowering the AC resistance in a high-frequency transmission line, the following Patent Literature 1 discloses a high-frequency wiring board in which the surface roughness (arithmetic mean roughness Ra) at an interface between a high-frequency wiring layer (transmission line) and a dielectric substrate is 0.3 μm or less. This high-frequency wiring board suppresses irregularities on a surface of the high-frequency wiring layer in contact with the dielectric substrate, so as to reduce reactivity at the interface and decrease the length of the transmission line, thereby lowering transmission loss.    Patent Literature 1: Japanese Patent Application Laid-Open No. 2001-015878